(i) formation of haploid cells, the ga­metes, by gametogenesis (meiosis)

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(ii) fusion of the two gametes forming diploid cells, the zygotes (fertilization)

(iii) repeated mitotic divisions of the zygotes to form embryos (em- bryogenesis); and

(iv) growth of embryos into new individuals (development). Because there is fusion of male and female gametes, the offspring produced are not identical to their parents or fellows.

Characteristics of Sexual Reproduction:

(i) It is usually biparental.

(ii) Gametes are always formed,

(iii) Fertilization takes place,

(iv) It involves both meiosis and mitosis,

(v) Daughter organisms genetically differ from the parents,

(vi) Multiplication is not so rapid as in asexual reproduction.

Origin of Sex:

Sex originated in protistans and simple algae. During favourable conditions, these organisms multiply asexually but during unfavourable conditions gametes are formed. The gametes fuse to form zygotes which often develops a thick wall to become zygospores. The latter are dispersed.

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Under favourable conditions zygospore germinates to form new organisms (e.g., prostist/alga). The zoospores (asexual spores) and gametes are structurally similar in Chlamydomonas and Ulothrix (both are simple algae).

Early sexual reproduction was isogamous. Later on evolution occurred and sexual reproduction became anisogamous and oogamous. Similarly sex organs evolved which differentiated into male and female individuals. Early evolution of sex organs depended upon environment but later on hormones started influencing the development of sex organs. In Chlamydomonas sexual reproduction may be isogamous, anisogamous or oogamous. (To be described ahead).

Occurrence:

Sexual reproduction occurs almost in all types of plants and animals.

Types:

Sexual reproduction is of two main types; syngamy and conjugation.

A. Syngamy (Gk. syn – together, gamos – marriage):

It is the complete and permanent fusion of male and female gametes to form the zygote.

Syngamy is of two types with regard to the source of fusing gametes; endogamy and exogamy.

1. Endogamy (Self-fertilization):

It involves the fusion of male and female gametes of the same parent. Thus it is uniparental, e.g., Taenia (tape worm). Taenia is hermaph­rodite (= monoecious or bisexual) worm.

2. Exogamy (Cross-fertilization):

It involves the fusion of two gametes produced by different parents. Thus it is bi-parental, e.g., Rabbit — dioecious or unisexual animal. Cross fertilization also occurs in many hermaphrodite animals as in earthworm and leech. It is due to the fact that their male and female reproductive organs mature at different times.

Syngamy is of following types with regard to the structure of the fusing gametes; isogamy, anisogamy (heterogamy), oogamy and hologamy.

1. Isogamy (Gk. iso = equal; gamos = marriage):

It involves the fusion of gametes which are similar morphologically but may be different physiologically. Such gametes are called isogametes. Isogamy takes place in Chlamydomonas – an alga and Monocystis – a protozoan.

2. Anisogamy (Heterogamy):

It involves the fusion of gametes which differ in size or motility. Such gametes are called anisogametes or heterogametes (e.g., microgametes or male gametes and macrogametes or female gametes. Anisogamy (Gk. аn-without, wo=equal, gamos=marriage) or heterogamy (Gk. hetero=different, gamos=marriage) occurs in Clamydomonas, some other algae, higher invertebrates and all vertebrates including human beings.

The common type of anisogamy is oogamy which involves the fusion of a large non- motile female gamete (egg or ovum) and a small motile male gamete (antherozoid).Thus in oogamy gametes differ in both properties. It takes place in Chlamydomonas, red algae, etc.

3. Microgamy:

The fusing gametes or individuals are much smaller than the normal. Examples— certain protistans.

4. Autogamy:

The fusing gametes/nuclei are derived from the same cell. Example — Paramecium.

5. Paedogamy:

It is a type of autogamy where gametes are derived from multiple division of the nucleus of the single individual. It differs from paedogenesis (reproduction by larva).

It involves the fusion of two organisms. It means two organisms themselves act as gametes. It occurs in yeasts.

B. Conjugation:

It involves temporary union of two parents of the same species which exchange their male pronuclei to form synkaryon and then separate to produce daughter individuals. It corresponds to cross fertilization of higher animals. It takes place in Parame­cium, Spirogyra, etc.

Advantages of Sexual Reproduction:

(Why is sexual reproduction better than asexual reproduction?)

1. Variations:

Since fusion of gametes from different parents occur during sexual reproduction, hence genetic recombination takes place causing variations.

2. Evolution:

Variation being a major factor of natural selection, therefore, it plays an important role in evolution.

3. Adaptation:

The offspring produced due to sexual reproduction adapt better to the changing environmental conditions.